Plywood manufacture



United States Patent 3,489,633 PLYWOOD MANUFACTURE Howard W. Holmquist,Seattle, Wash., assignor to Monsanto Company, St. Louis, Mo., acorporation of Delaware No Drawing. Filed Dec. 30, 1966, Ser. No.606,005 Int. Cl. C09 02; B32b 31/04 US. Cl. 156-312 3 Claims ABSTRACT OFTHE DISCLOSURE This invention relates to the manufacture of hotpressedplywood and particularly to the pro-press of a sembled panels.

In the manufacture of hot-pressed lywood, core stock veneers are wetspread with an adhesive composition and assembled with uncoated veneersto form a panel comprising a plurality of veneers in which the woodgrains of adjacent veneers are disposed at right angles. The panel issubsequently hot-pressed to set up the adhesive and bond the veneerstogether. In conventional plywood mill procedure, a plurality of panelsis laid up and stacked one on top of the other to form a press loadcontaining from about 20 to 30 panels depending upon the capacity of thehot-press. Pre-press is the technique whereby the stacked panelassemblies are subjected to a preliminary cold pressure period prior toseparation and loading into individual hot-press openings for finalconsolidation and cure. Pre-press is thus intended to establish atemporary bond of sufficient strength to facilitate panel handlingbefore hot pressing. This is of commercial value because it (a) allowsautomatic press loading, (b) the consolidated panels can be loaded intosmaller press openings which allow more platens for a given press frame,and (c) it significantly reduces panel rejects due to veneermisalignment during hot-press loading and closing opera tions. Theadhesive is applied to the core stock as a mobile liquid and must beallowed to lose moisture through absorption or evaporation before a coldpressure pre-press bond can be established. Accordingly, the press loadis allowed to stand for a period of time after last panel lay up. Atypical mill pre-press cycle comprises about a 3 minute stand time afterthe last panel is assembled and then the press load is compressed for 4minutes. Such pre-press cycle develops satisfactory pre-press bonds inthe lower panels of the load but the upper panels still delarninate andadversely affect subsequent handling and loading of the hot-press. Sincethe lay-up of a press load requires from to minutes, the first panelsassembled have considerably more time for the adhesive spread to losemoisture and become tacky than do the latter assembled panels.Therefore, it would be highly desirable to equalize the tack level ofthe adhesive throughout a press load of panels Without disrupting thenormal mill assembly times or hinder the hot-press bonds.

Accordingly, typical objects of this invention are to provide (1)improved pro-press bond strength of assembled plywood panels, (2)improved method for prepressing assembled plywood panels, and (3)improved method for manufacturing hot-pressed plywood.

Other objects, aspects and advantages of this invention will becomeapparent to those skilled in the art upon further study of thisdisclosure and the appended claims.

3,489,633 Patented Jan. 13, 1970 In accordance with this invention, theimproved technique for pro-pressing a press load of plywood panelscomprises the steps of (a) applying a first cold compressive pressure tothe press load immediately after the final panel lay-up for a period oftime ranging from 30 to seconds, (b) releasing the pressure and allowingthe press load to stand uncompressed for a period of time ranging from 2to 5 minutes, and (c) applying a second cold compressive pressure to thepress load for a period of time ranging from 30 to 90 seconds. Thepre-press bonds obtained by this technique are uniform throughout thepress load and stronger than those obtained by the present pre-pressingtechniques.

Any of the adhesive compositions useful in the manufacture of plywoodcan be used as the adhesive composition when practicing this invention.Such adhesives are well known in the art and preferably contain anaqueous alkaline phenol-formaldehyde condensate. The customaryrelatively lengthly stand times of commercial adhesives required toachieve acceptable p-re-press bond strengths is considerably reduced bythe present invention while still achieving superior pro-press bondstrengths and maintaining the time tolerances of a plywood mill.

The following examples are given to illustrate the invention and suchare not intended to limit the scope of the invention concept.

EXAMPLE I A medium grain Douglas Fir veneer is cut into 4%" x 5%"panels. Pairs of the panels are matched and one surface of one panelfrom each pair is spread with about 70 pounds per MDGL (thousand squarefeet of double glue line) of an adhesive composition prepared asfollows:

Parts by weight Water at 70 F. 438

Unrefined potato starch 50 Mix two minutes 50% caustic soda solution 9Mix six minutes Furafil 1 200 Mix five minutes 50% caustic soda solution48 Mix 15 minutes Alkaline phenol formaldehyde resin 2 1250 Mix fiveminutes and cool the mixture to room temperatu-re.

A solid furfural by-product residue from the acid hydrolysis of apentosan containing material selected from the group consisting ofground oat hulls and ground corn cobs, said residue substantially 'allpassing a mesh standard soreen.

23 3r27resin as prepared in accordance with U.S. Patent Re.

The second panel of each pair is then placed across the coated surfaceof the first panel with its grain running at a 90 angle to that of thefirst panel and with the long dimension of each panel over-lapping theshort dimension of the other panel on all sides. Each panel assembly ispermitted to stand a short time (assembly time) after layup to simulatemill procedure. The panel is then cold pressed at 70 F. and psi.pressure (pre-press). The pressed assembly is then broken on alaboratory puller having a fixed base jig under which the overlappingends of one panel are placed and a second, movable jig attached to ascale under which the overlapping ends of the second panel are placed.The assembly is then pulled apart, using a load rate of about 10 poundsper second, recording the pounds of pull at the break for each assembly.The comparative results of a conventional pre-press cycle and that ofthe instant invention are given below in Table I.

TABLE I CONVENTIONAL FEE-PRESS CYCLE Assembly Adhesive Percent Percent Tme Pro-Press Wet Spread Adhesive Wood (mun) Time (min) (MD GL) Lbs. PullTransfer Failure DUAL PRE-P RESS CYCLE Second First Aeration PressureAdhesive Percent Assembly Pressure Time Period Wet Lbs. Percent WoodTime (111111.) (min.) (nun) Spread Pull Transfer Failure 1 min. 1 1 3 168 110 100 0 30 sec 1 2 3 1 70 60 100 0 A soft grain southern pineveneer thick is cut into i a series of 4%" x 5 /8" panels. Pairs of thepanels are matched and one surface of one panel from each pair isadhesive can be increased by forced air circulation. The second coldpressure period consolidates the pro-press bond throughout the load.

What is claimed is: 1' 1. The method of pre-pressing plywood panelswhich comprises the steps of:

(a) applying a first cold compressive pressure to the panels for about30 to about 90 seconds; (b) releasing said pressure for about 2 to about5 mincoated with a commercial adhesive composition. The pairs 25 ofveneers are placed together and tested as outlined in mes and anowmgSald Panels to expand vertically; Example I. Comparative data includingassembly and prei I press times and adhesive spread levels are givenbelow in pplymg a Second cold compressive pressure to the Table II.panels for about 30 to about 90 seconds.

TABLE II CONVENTIONAL CYCLE Pre- Assembly Press Adhesive Percent TimeTime Wet Spread Adhesive (min) (min) (MD GL) Lbs. Pull Transfer PercentWood Failure 2 4 98 100 0 (wet glue line). i 4 90 100 0. 7 5 96 60 95 0(some adhesive tack). 15 5 96 100 95 0.

DUAL PRESSURE CYCLE Adhesive Assembly First Open Second Wet Percent TimePressure Time Pressure Spread Lbs. Percent Wood (111111.) (see) (min.)(min). (MD GL) Pull Transfer Failure This example shows the variation inpro-press bond strength throughout a press load of panels and theimproved bond strengths obtained by this invention.

From the preceding disclosure, those skilled in the art will realizethat the initial cold pressure period provided will (1) establishsatisfactory pre-press :bonds in the lower portion of the load becausethose panels have had 10 to 15 minutes stand time and the adhesive is ina tacky stage and (2) will transfer about half of the adhesive fromthespread cores to adjacent veneers in the upper portion thus causing theadhesive to develop tack at a faster rate through greater adsorptioninto the wood. The early establishment of transfer in the lower portionof the load will also tend to improve the final hot press adhesive bond.A normal pre-press cycle which holds the load 3 or 4- minutes beforepressure may allow the bottom panels to become so dry they will nottransfer and properly bond the adjacent plys during hot press. When theinitial pressure is released, the upper portion of the load expandsvertically due to some warpage of the veneers. This opening of theveneers allows for the adhesive to be aerated and lose moisture. Theevaporation of moisture from the 2. The method of claim 1 wherein aplurality of panels are pre-pressed.

3. In the manufacture of plywood wherein a press load of panels isassembled pre-pressed and subsequently hotpressed, the improvement whichcomprises applying a first cold compressive pressure to the loadimmediately after the final panel lay-up for about 30 to about seconds,releasing said pressure and allowing said panels to stand for about 2 toabout 5 minutes, and applying a second cold compressive pressure to theload for about 30 to about 90 seconds.

References Cited UNITED STATES PATENTS 1,125,973 1/1915 Davidson.1,329,402 2/ 1920 Henning.

REUBEN EPSTEIN, Primary Examiner US. Cl. X.R.

